It has previously been proposed (see European Patent Application No. 0 107 627, Ballarini) to construct a chair of multiple back segments. As shown in this reference, five stacked segments are used, connected by four horizontal connections, which form rotary axes. The chair back is bulged slightly towards the rear. When the user leans back, the various segments rotate about their axes, thus increasing the bowing of the back, giving a cradling effect. The shafts about which the axis of rotation occurs are restrained by springs which have the tendency to return the back into its normal position. The back is bowl-shaped, to cradle the user. Thus, the joints which connect the respective segments must be located essentially in the center of the back construction. Lateral arrangement is only possible if the axes of rotation extend parallel to each other. Such an arrangement, however, substantially limits the freedom of the designer in constructing the chair and becomes very costly. Further, the stability of the back is impaired.
It is possible to connect the various back sections by separate rotary joints; it is also possible, however, and as shown in this publication, to connect the segments by linked spring elements in the form of torsion rods or bars. The torsion rods or bars extend from one side of the back to the other, and are retained, rotatably, in the center of the back. The ends of the torsion rods or bars have oppositely bent-over parts secured thereto, for attachment to the respective back segments. To attach two segments, five attachment elements or arrangements are necessary, four for the ends of the torsion rods and one for the center, an expensive and complex arrangement.
Adjustment of the spring pressure is not possible in these arrangements. Thus, the designer is faced with always designing for a compromise. If the user is light-weight, the back is too stiff. If the user is quite heavy, it is too flexible, and does not provide the necessary support; on the contrary, it may lead the user to have a feeling of instability. For a heavy user, the segmented back is less desirable than a single unit back; for a light person, the segmented back is useless and merely excessively complex and expensive.